1. Field of the Invention
This invention relates to electrical protection systems.
2. Introduction to the Invention
The parent application, now U.S. Pat. No. 5,831,803, discloses electrical protection systems. Such systems can be connected between an electrical power supply and an electrical load to form an operating circuit. When so connected, such systems protect the circuit from overcurrents. The disclosed systems have a normal operating condition and a fault condition. Such systems include a current sensing means, such as a current relay coil having a current deenergized state when the current passing through it does not exceed a normal current, I.sub.NORMAL, by a predetermined current amount; and, a current energized state when the current passing through it has exceeded the normal current, I.sub.NORMAL, by the predetermined current amount. The systems further include a first circuit switch coupled to the current sensing means (relay coil) which is closed in the current deenergized state and open in the current energized state. The systems further include a voltage sensing means, such as a voltage sensing relay coil, having a voltage deenergized state when the voltage across the voltage sensing means does not exceed a predetermined voltage amount, and a voltage energized state when the voltage exceeds the predetermined voltage amount. The systems also include a PTC device connected in parallel with the first switch, and a second circuit switch. The second circuit switch is coupled with the voltage sensing means and is closed in the voltage deenergized state and opened in the voltage energized state. The second circuit switch is connected in series with the parallel combination of the first circuit switch and the PTC device. The voltage sensing means is connected in parallel with a series combination of the second switch and the parallel combination of the first circuit switch and the PTC device.
The described systems are similar to the system shown in FIG. 2c of U.S. Pat. No. 5,689,395 with the advantage that opening of the second circuit switch transfers the system voltage to the voltage sensing means. Therefore, the arrangement of the disclosed systems of the parent application ensures that the second circuit switch continues to open and remain open without relay chatter.